With the impending energy crisis and increasing environmental concerns, a worldwide imperative is to develop greener and more efficient energy conversion and storage devices with the goal of utilizing renewable and sustainable energy sources, such as solar, wind, geothermal and tidal energy. Lithium-ion batteries, as one of the most important energy storage devices, have revolutionized the modern society in all aspects, especially for consumer electronics and electric vehicles, because of their high energy density, light weight and long lifespan. To date, graphite is frequently used as an anode material, however, its low theoretical capacity (372 mAh g−1) and unresolved safety issue hinder its practical application, especially for large-scale grid energy storage and sustainable transport. Many efforts have been made to develop electrode materials with superior capacity and longer lifespan to replace graphite. In this context, numerous anode materials including silicon, germanium, tin, transition metal oxides (i.e. Co3O4, SnO2, NiO) and sulfides (i.e. NiS, Ni3S4, SnS2, CoS2, Co9S8, FeS2) have been extensively explored as alternative anode materials, due to their remarkable theoretical capacities, high lithium activity, low cost and natural abundance. However, metal oxides and sulfides usually suffer from huge volume change and pulverization during Li+ insertion-extraction process, which often lead to cracking, poor electronic conductivity or even fracture of the anode.
Compared with metal oxides, metal sulfides possess combinatory physical and chemical properties, such as higher electrical conductivity, better thermal stability, and richer redox chemistry, making them more promising for lithium-ion batteries. Among various metal sulfides, nickel sulfides have been well explored in a wide range of technical fields, including solar cell, hydrogen production, catalysis, optoelectronics, energy conversation and storage. For instance, NiS, NiS2, Ni3S2, and Ni3S4 have gained special attention as electrode materials for lithium-ion batteries and supercapacitors.
A bulk electrode usually exhibits sluggish lithium ion diffusivity because of its low active surface area, which limits the inner capacitive contribution from active materials, thereby leading to poor power and energy density. Many efforts have been made to enhance the capacity and extend lifespan of lithium-ion batteries by either constructing nanostructures of metal sulfides on conductive porous carbon matrix or coating carbon onto metal sulfide nanoarchitectures. Recent studies have shown that the design wisdom of nanostructured active materials on conductive porous backbone (usually carbon) indeed pushes up energy and power outputs.
Many efforts have been made to develop safe, lightweight and flexible power sources to meet the urgent need for flexible/wearable electronics. Graphene has been recognized as an ideal substrate to hybridize active materials for high-performance energy storage devices because of its large surface area, ultra-high electrical conductivity, high flexibility, outstanding mechanical properties, and eminent chemical stability. To date, advances have been made in the synthesis of graphene-based, hybrid active materials for energy storage. The graphene in those hybrid electrodes serves as both conductive matrix and building blocks, forming a superstructure with interconnected conductive networks, which in turn facilities Li+ ion transportation.